Listed Ford transmissions may exhibit no line rise and/or line pressure instability due to a broken isolator valve spring. The isolator valve is in the main line pressure circuit and acts as a damper to smooth pulses from the pressure control solenoid. When this spring breaks, the end plug can shift toward the middle of the valve body, allowing unrestricted exhaust for main line pressure. Spring breakage can also cause isolator valve to jam in the bore, preventing the dampening effect that protects against line pressure instability. Installing Sonnax isolator valve spring 124740-16 restores normal line pressure operation. The spring is shot-peened to reduce spring fatigue and eliminate premature failure.